The icosahedral borides, such as boron phosphide (B12P2) and boron arsenide (B12As2), are hard and chemically inert solids that exhibit exceptional radiation tolerance due, at least in part, to the strong bonding within the boron icosahedra. It has been suggested that if these wide bandgap materials could be suitably doped, they would be useful for a variety of applications, in particular those applications requiring radiation hardness and/or high temperature capabilities. Early work has indicated that a high background impurity concentration will degrade the luminescence properties of B12P2 while crystalline imperfections are expected to degrade the electrical transport properties of the material. It is expected that B12As2 and B12P2 will exhibit similar electrical and optical behavior because of the structural similarity of these two materials.
Crystalline perfection and background impurity issues are linked as crystalline imperfections cause increased contamination incorporation through accelerated diffusion. Additionally, crystalline imperfections provide natural locations for accommodating such contaminants. Therefore it is anticipated that the intrinsic electrical, optical and other properties of B12P2 and B12As2 will best be revealed in high crystalline quality samples that have a low background impurity concentration.
In order to obtain the desired icosahedral boride material, a number of parties have produced B12P2 and B12As2 thin films using chemical vapor deposition (CVD) techniques. For example, in 1973 Hirayama et al. published a note entitled “Hetero-Epitaxial Growth of Lower Boron Arsenide on Si Substrate Using Ph3—B2H6—H2 System” (Jap. J. Appl. Phys., 12 (1973)1504-1509) in which it was shown that B12As2 could be deposited using dilute hydride sources of diborane (B2H6) and arsine (AsH3) in a hydrogen ambient environment. The B12As2 films were deposited on silicon substrates with three different orientations, (100), (110) and (111). The film morphology was found to be orientation dependent. Electron reflection diffraction analysis indicated that the films were single crystal, epitaxial B12As2 thin films containing patches of polycrystalline material.
Years later, in an article entitled “Chemical Vapor Deposition of Boron Subarsenide Using Halide Reactants” (Reactivity of Solids, 2 (1986)203-213), Correia et al. demonstrated that B12As2 films could be grown by CVD on a variety of substrates (i.e., tungsten, nickel, fused quartz, Si(111) and Si(100)) using the halide sources BBr3 and AsCl3. The authors established that the film crystallinity was dependent on growth conditions, especially growth temperature and source flow rate, and showed how changing these conditions could yield either amorphous films or polycrystalline films. They also found that during deposition on a silicon substrate, intermixing occurred between the B12As2 and silicon, with up to 4% Si being found in the B12As2 film.
In 1997 Kumashiro et al. published an article entitled “Epitaxial Growth of Rhombohedral Boron Phosphide Single Crystalline Films by Chemical Vapor Deposition” (J. Solid State Chem., 133 (1997)104-112) reporting the results of B12P2 film growth on silicon using CVD techniques. The authors confirmed the sensitivity of film crystallinity in B12P2 to the growth conditions and found that polycrystalline B12P2 was obtained at a growth temperature of 1050° C. while single crystal B12P2 was obtained at a temperature of 1100° C. They also confirmed earlier findings that reactant gas flow is the most important parameter in determining the quality of the grown crystal.
Although it appears that the growth conditions for B12As2 and B12P2 have been optimized, the desired film crystallinity and impurity concentrations have not yet been achieved. Accordingly, what is needed in the art is a method for achieving the desired film crystallinity and impurity concentrations in icosahedral boride materials. The present invention provides such a method and the desired resultant material.